I used the rare earth magnets that were about 3/4" long 1/2" wide and 1/16" thick approximately. 2 stacks about 5 high end to end. They will stick to the center shaft of the motor just fine. I would be careful about ordering 1 big magnet because if the polarity is wrong the stir bar will not center itself and fly off. Here's a link to some 1"x1"x 1/8" magnets that cost $12.99 for a four pack. 2 packs would be needed or you can glue a spacer onto the motor and just use a four pack. There is a little over 1/2" from the top of the unit to the motor.
Here's a link
Here's a link to the same stir bar I bought. I would like to try one that is 3/8" or 1/2" diameter but the 5/18 seamed to work ok. There is all kinds of shapes and sizes so pick what you want or find an assortment and have some fun.
The .090 thick lexan is available at Home Depot in a 1' x 2' sheet for around $24.00 Just be careful using a table saw because it feels like it is going to take flight when you start cutting. It may have been a dull blade but I just went slow and cut 2 pieces. I used the double sided 3m mount molding tape usually found at auto parts stores to hold the lexan on.
Here a linky